1. Field of the Invention
The present invention relates to a signal processing method in readout of signals from a magnetic recording medium, on which magnetic bits are recorded along the tracks thereof by using a magnetic transfer method. The present invention also relates to a magnetic record reproduction apparatus.
2. Description of the Related Art
Conventionally, a magnetic transfer method for transferring magnetized patterns corresponding to information carried by a master information carrier for magnetic transfer onto a magnetic recording medium (slave medium) is well known. In this method, the master information carrier for magnetic transfer, which carries the information to be transferred, represented by very fine uneven patterns, and the magnetic recording medium are placed in close contact with each other, and a magnetic field for transfer is applied to the master information carrier for magnetic transfer and the magnetic recording medium which are in close contact with each other. In this magnetic transfer method, recording may be performed statically without changing relative positions between the master information carrier and the magnetic recording medium. Further, the magnetic transfer method has an advantage that the information can be recorded in a very short time (for example, please refer to Japanese Unexamined Patent Publication No. 10(1998)-040544 and Japanese Unexamined Patent Publication No. 10(1998)-269566).
Application of this magnetic transfer technique to recording of servo signals for positioning a magnetic head on a magnetic recording medium has been already proposed in Japanese Unexamined Patent Publication No. 10(1998)-040544, Japanese Unexamined Patent Publication No. 10(1998)-269566, or the like.
However, the magnetic transfer method has a technical problem that when signals are transferred onto the magnetic recording medium from the master information carrier, an unclear magnetic recording area (reversely magnetized area) is produced on the magnetic recording medium. Since the unclear magnetic recording area is present on the magnetic recording medium, a sub-pulse is detected in a readout waveform of magnetized patterns. Therefore, there is a possibility that the sub-pulse is recognized as a reproduced signal by a magnetic reproduction apparatus, which may cause an error in reproduction of the signals. A method for observing the sub-pulse, generated in the unclear magnetic recording area, is disclosed in U.S. Patent Application Publication No. 20020044368, Japanese Unexamined Patent Publication No. 2002-042301, or the like.
When transferred signals are, for example, servo signals, if a space exceeding a length corresponding to three bits, in which magnetic bits are not recorded, is present on the magnetic recording medium after magnetic transfer, it is difficult to completely prevent generation of a sub-pulse caused by the unclear magnetic recording area. When the sub-pulse is present, tracking performance drops. An example will be described with reference to schematic diagrams illustrated in FIGS. 6A through 6C.
FIG. 6A is a schematic diagram illustrating a partial sectional view of a master information carrier in the track direction. FIG. 6B is a schematic diagram illustrating magnetized patterns on a corresponding portion of a magnetic recording medium, onto which magnetized patterns are transferred from the master information carrier illustrated in FIG. 6A. FIG. 6C is a schematic diagram illustrating a readout waveform which is detected from the magnetized patterns.
Uneven patterns based on desired information are formed on a master information carrier 101. The uneven patterns include protrusions and depressions which have various bit lengths. If the minimum bit length (length in the track direction) among the various bit lengths is Bmin, many spaces (depressions), each having a length L which is longer than or equal to a length corresponding to three bits, namely L≧3Bmin, are present. The master information carrier 101 includes a substrate 101a made of a magnetic material such as Ni, for example. The substrate 101a bears uneven patterns based on desired information on the surface thereof. The master information carrier 101 for magnetic transfer also includes a magnetic layer 101b which is formed on the surface of the uneven patterns of the substrate 101a. The surface of the master information carrier 101 for magnetic transfer, on which the magnetic layer 101b is formed, and a magnetic recording layer of the magnetic recording medium, on which direct current magnetization has been carried out in advance in a single direction along the track, are placed in close contact with each other. While they are in close contact with each other, a magnetic field for transfer is applied in a direction opposite to that of the direct current magnetization. Accordingly, the magnetized patterns as illustrated in FIG. 6B are recorded on the magnetic recording medium. At this time, unclear magnetic recording areas 104 are generated at both ends of an area corresponding to a space which has the aforementioned length L. Further, a readout waveform as illustrated in FIG. 6C is detected from the magnetized patterns illustrated in FIG. 6B. As illustrated in FIG. 6C, sub-pulses 105 which correspond to the unclear magnetic recording areas 104 are detected. Specifically, the sub-pulses are detected at a reproduction apparatus for reproducing data from information recorded on a magnetic recording media. Therefore, there is a possibility that the sub-pulses are incorrectly recognized as reproduced signals. For example, when sub-pulses are generated at both ends of space portions of burst signals in servo patterns, the timing of the beginning and end of the servo signal area is incorrectly recognized. Therefore, the servo timing is shifted, and there is a possibility that an error such as failure in servo tracking is caused.